This proposal describes five projects which focus on the anatomy, physiology and neurochemistry of the medial, lateral and sympathetic efferent innervation of the cochlea in developing and adult animals. Development is an additional common element in three of the five projects which focus on cell number regulation in the efferent system, pathfinding mechanisms of efferent axons, synaptogenesis and postsynaptic receptor maturation, and functional consolidation of efferent connections. The first project will continue to investigate the detailed anatomy of different subcomponents of the olivocochlear system. The second project will quantify numerical matching of efferent cells to their targets in control and mutant mice with different degrees of reduction in the periphery. The third project will continue to assess the physiological development of efferent function, focusing on the effect of early chronic differentiation. The fourth project will focus on the neurogenic regulation of cochlear blood flow using a new in vitro preparation of the spiral modiolar artery and will determine the functional interactions between the sympathetic efferent innervation and its target. The fifth project will continue to study the neurochemistry of the efferent system, focusing on cholinergic receptor subunits in adult and developing rats.